A light-emitting element using a light-emitting material has advantages of thinness and lightweight, high-speed response, DC low-voltage drive, and the like and is expected to be applied to a next-generation flat panel display. In addition, it is said that a light-emitting device having light-emitting elements disposed in matrix is superior to a conventional liquid crystal display device in a wide viewing angle and high visibility.
A light-emitting element is said to have the following light-emission mechanism: voltage is applied to a light-emitting layer sandwiched between a pair of electrodes, electrons injected from a second electrode and holes injected from a first electrode are recombined in a light-emission center of the light-emitting layer to form molecular excitons, and then light is emitted by releasing energy when the molecular exciton returns to the ground state. A singlet excited state and a triplet excited state are each known as the excited state and the light emission is considered possible via any one of the excited states.
In order to enhance the characteristic of such a light-emitting element, the improvement of the element structure, the development of the material, or the like is performed.
For example, a method in which an optical length L from a light-emitting portion to a reflective electrode is controlled by sandwiching ITO between the light-emitting portion and a reflective metal is given as a means for increasing the external quantum efficiency without deteriorating the luminance by controlling the distance from the light-emitting region to the reflective metal (see, for example, Reference 1: Japanese Patent Application Laid-Open No.: 2003-272855).
FIG. 2 schematically shows an element structure disclosed in Reference 1. In this structure, a transparent electrode 201, a light-emitting portion 202, a transparent conductive film 203, and a metal electrode 204 are stacked. By adjusting the thickness of the transparent conductive film 203, the optical length L from the light-emitting portion to the metal electrode is optimized to increase the external quantum efficiency.
However, according to the structure disclosed in Reference 1, since the transparent conductive film 203 and the reflective metal (metal electrode) 204 are in contact, there is a problem of erosion. Here, it is known that the erosion might occur due to the difference in their self-potential or the like, and also the erosion is referred to as electric erosion (see, for example, Reference 2: Japanese Patent Application Laid-Open No.: 2003-89864). Reference 2 describes the self-potential measured using a sodium chloride solution of 3.5% (liquid-temperature of 27° C.) and using silver/silver-chloride as a reference electrode. In the case of such a measurement, the self-potential of aluminum known as metal having high reflectivity is approximately −1550 mV, while that of ITO serving as a transparent conductive film (In2O3 containing SnO2 by 10 wt %) is approximately −1000 mV. Thus, the difference between these self-potentials of aluminum and ITO is large. Therefore, when aluminum and ITO are stacked to be in contact with each other, it is very likely that oxidation-reduction reaction progresses at a stacked interface between aluminum and ITO, which highly results in electric erosion. Such a problem of electric erosion is generated regardless of the combination of ITO and aluminum.
The self-potential is potential of a reaction to a reference electrode when the reaction is soaked in a certain solution in such a state that current is not applied from outside, that is, potential in a closed loop and is also referred to as resting potential.